Communication systems typically comprise a transmitter connected to a receiver via a communication link (referred to as a primary link). In most communication systems, intersymbol interference (ISI) is a common channel impairment that results from the transmitted pulses spreading beyond their allotted time intervals and interfering with neighboring pulses, thereby degrading the performance of the system. Other channel impairments include non-linear effects (e.g. intermodulation distortion) from radio frequency (RF) components in the transmit chain and receive chain. Two common methods of combatting these channel impairments are forward error-correcting codes and equalization.
An equalization approach typically involves filtering the received signal to cancel the channel impairments introduced by the channel. Specifically, an estimate of the channel impulse response is generated at the receiver and used to mitigate the effect of the channel.
Typically, the channel estimate is generated by using pilot or training symbols (known at both the transmitter and receiver), which are transmitted along with data symbols, as shown in FIGS. 1A and 1B. For example, FIG. 1A shows two packets in a time-domain waveform, wherein the payload of each packet is preceded by a set of training symbols. At the receiver, the training symbols are used to generate a channel estimate, which is then used to equalize the subsequent payload. Although training preambles are used, there has always been an effort to minimize their use in order to maximize throughput and minimize the end-to-end latency.
Most communication channels vary with time, but the time over which the channel is considered to be quasi-static is defined as the coherence time of the channel. As shown in FIG. 1A, a training portion must precede each payload portion in a packet since a channel estimate derived for a specific packet cannot be used for the effective equalization of several subsequent packets, since the subsequent packets will typically arrive after the coherence time of the channel has elapsed, and the channel estimate will no longer be representative of the channel.
FIG. 1B shows a time/frequency allocation diagram for an OFDM system; specifically, the subcarriers are shown for an OFDM frequency-domain waveform as a function of time. It is commonplace to use pilot symbols in certain subcarriers to enable the equalization of the data subcarriers, and one possible arrangement of pilots is shown in FIG. 1B. The pilot symbols are used at the receiver to derive a channel estimate, which is subsequently used to equalize the data subcarriers. In both of the scenarios shown in FIGS. 1A and 1B, pilot and training symbols are overhead that reduce the system throughput over the wireless link being used.
More recent OFDM systems employ multiple antennas at both the transmitter and the receiver (MIMO-OFDM), and employ a variety of methods including pilot-aided and data-aided channel estimation in order to equalize the channel at the receiver. As described above, these systems are typically designed to maximize the throughput, or minimize the bit- or packet-error rate or end-to-end latency, over the point-to-point link between the transmitter and the receiver.